http://d.puremagic.com/issues/show_bug.cgi?id=11148

           Summary: Can't implicitly convert const(BigInt) or
                    immutable(BigInt) to BigInt
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: joseph.wakeling@webdrake.net


--- Comment #0 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-09-30 10:45:06 PDT ---
Created an attachment (id=1253)
Can't pass const(BigInt) to a function taking a BigInt argument.

const(BigInt) or immutable(BigInt) objects can't be assigned/implicitly
converted to mutable BigInt values.  The attached code gives examples where a
const or immutable BigInt is passed to a function taking a (mutable) BigInt
argument.

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--- Comment #1 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-09-30 10:45:39 PDT ---
Created an attachment (id=1254)
Can't pass immutable(BigInt) to function taking a BigInt argument.

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--- Comment #2 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-10-08 09:04:03 PDT ---
Created an attachment (id=1260)
Third example, which tests implicit, explicit and cast-based conversion

In fact, there is no reliable way to convert from a qualified BigInt type to an unqualified one.  The 3rd code example attached shows attempted conversion through both implicit conversion (simple assignment) and explicit (trying both std.conv.to and cast).

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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
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                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #3 from hsteoh@quickfur.ath.cx 2013-10-10 11:15:45 PDT ---
Is this really a bug? BigInt is implemented as a reference type, so allowing a cast from const(BigInt) to BigInt would be a bug.

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--- Comment #4 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-10-10 11:27:44 PDT ---
I think it's an issue that you can't pass an immutable(BigInt) to a function that accepts a BigInt as input.  This makes it difficult to e.g. write generic functions that work with any integer type.

I'm not sure what the appropriate way to deal with that is, though.

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--- Comment #5 from hsteoh@quickfur.ath.cx 2013-10-10 11:39:18 PDT ---
You can't pass an immutable(BigInt) to a function that takes unqual BigInt,
because that breaks the type system (and risks breaking immutability). The
function needs to take const(BigInt) if it doesn't need to propagate qualifiers
to its return type, or inout(BigInt) if it does.

If the function needs to modify the BigInt, then it should rightly reject any attempt to pass in an immutable BigInt.

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--- Comment #6 from Joseph Rushton Wakeling <joseph.wakeling@webdrake.net> 2013-10-10 11:55:30 PDT ---
(In reply to comment #5)
> If the function needs to modify the BigInt, then it should rightly reject any attempt to pass in an immutable BigInt.

Well, suppose you have some function of the form,

    T foo(T)(T n)
    {
        Unqual!T a = n;
        // ... do mutable stuff with a
        return a;
    }

That still won't work if T is an immutable(BigInt), because the first line will fail: you can't copy from the immutable to mutable.

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Brad Roberts <braddr@puremagic.com> changed:

           What    |Removed                     |Added
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                 CC|                            |braddr@puremagic.com


--- Comment #7 from Brad Roberts <braddr@puremagic.com> 2013-10-10 16:28:11 PDT ---
BigInt must behave essentially exactly like every other integral type (except for having a larger range of possible values), which includes being a value type.  Any internal COW implementation must be invisible to users.

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Simen Kjaeraas <simen.kjaras@gmail.com> changed:

           What    |Removed                     |Added
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           Keywords|                            |pull
                 CC|                            |simen.kjaras@gmail.com


--- Comment #8 from Simen Kjaeraas <simen.kjaras@gmail.com> 2013-11-01 18:29:34 PDT ---
https://github.com/D-Programming-Language/phobos/pull/1679

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